Atrio-Ventricular Septal Defect

Atrioventricular Canal Defect (AVCD), often interchangeably referred to as Atrioventricular Septal Defect (AVSD) or Atrial Ventricular Septal Defect, describes a spectrum of congenital heart abnormalities. This condition represents a defect in the formation of the mesenchyme, specifically the superior and inferior cushions, which develop abnormally. This abnormal development hinders the common atrioventricular (AV) valve from properly dividing into distinct tricuspid and mitral valves. A consistent feature across all forms of AVCD is the presence of a common annulus or junction. This common annulus can lead to the failure of the atrioventricular septum to develop fully, resulting in missing components of both the atrial and ventricular septa in the middle of the heart. The embryo naturally starts with an atrioventricular canal defect, where superior and inferior cushions are evident, and a common AV valve sits entirely over the left ventricle. The progression beyond this embryonic stage is crucial for normal heart development. AVCDs are categorized based on the extent of the septal defect and valve attachments. Regardless of the classification, the common annulus or junction remains a unifying anatomical feature.

The Rastelli classification of atrioventricular septal defect (AVSD) categorizes the anatomy of the common AV valve based on how the chordal attachments of the valve relate to the ventricular septum. This classification is important for surgical planning and determining the complexity of repair.

Rastelli Types of AVSD:

1. Type A (Most Common)

   • The superior bridging leaflet is split at the level of the ventricular septum.

   • The left superior (anterior) leaflet (LSL) remains entirely within the left ventricle, while the right superior (anterior) leaflet (RSL) is positioned exclusively within the right ventricle.

2. Type B (Rare)

   • The superior bridging leaflet is divided and attaches to a papillary muscle in the right ventricle.

   • The abnormal papillary muscle attachment originating from the right side of the ventricular septum connects to the left portion of the common superior (anterior) bridging leaflet.

   • This type is extremely rare and is often associated with heterotaxy syndromes.

   • Surgical repair can be more complex due to abnormal chordal distribution.

3. Type C - Click here for example

   • The superior bridging leaflet remains intact and free-floating, without septal attachment. This type features significant bridging of the ventricular septum by the superior (anterior) bridging leaflet (SL), which remains intact and does not divide. It is free-floating without any chordal attachments to the septal crest. The posterior common leaflet may either be divided or undivided, but in most cases, it is well anchored.

   • This leads to a large communication between the left and right ventricles.

   • Surgical correction is more challenging, requiring careful reconstruction of the left AV valve to ensure competent function.

Partial AVSD

• Also known as incomplete AV canal or ostium primum ASD. These patients have no VSD component because the atrial ventricular valve is densely adherent to the ventricular septum. This adherence divides the common annulus into two orifices. The physiology is primarily that of an atrial septal defect, with shunting only at the atrial level. Repair can be performed electively, typically between 2 to 5 years of age.  Variations include cases where the AV valve is densely adherent with no tissue visible underneath, or where an aneurysm of AV valve tissue has closed off a potential VSD.

• Primum ASD: This defect is located in the lower atrial septum, just anterior to the coronary sinus orifice. A key echocardiographic feature is that the atrioventricular (AV) valves attach to the interventricular septum (IVS) at the same level. As with all atrial septal defects, assessing the degree of right ventricular volume overload (RVVO) is essential.

Transitional Form: 

Considered an intermediate type between complete and partial forms. There are some attachments of the AV valve to the ventricular septum, but not enough to completely close it off, resulting in a restrictive ventricular septal defect. The timing of repair depends on the size of the VSD. Clinical presentation can be problematic even with a small VSD, if there's significant atrial shunting causing high pulmonary blood flow and failure to thrive, necessitating early intervention (e.g., at 2.5 months).

Complete AVSD

• Characterized by a large atrioventricular septal defect with an unrestricted ventricular septal defect (VSD) component and typically a very large primum atrial septal defect (ASD) component. Physiologically, these patients experience early heart failure due to the large VSD. Repair is usually required at a young age, typically between 2 to 6 months. ◦ In the complete form, the common AV valve sits somewhat in the middle of the defect, allowing for both atrial and ventricular level shunting.

• Intermediate Type: This is a complete form where the common valve is actually divided, with superior and inferior leaflets connected, rather than a single orifice. A common annulus is still present. Repair timing is similar to the complete common AV canal due to the VSD component's size.

• Balanced AVSD: This condition involves both an atrial and ventricular septal defect (VSD). The term "balanced" refers to the relative size of the ventricles, particularly the inlet portion (determined by the size of the AV valves). Given that AVSDs are often part of complex congenital heart disease, it is crucial to evaluate systemic and venous return. Additional anomalies to assess include:

  • Presence of additional VSDs.

  • Patent ductus arteriosus (PDA).

  • AV valve regurgitation, with attention to the origin of the regurgitant jet.

  • AV valve morphology, classified using the Rastelli system (Types A, B, and C), best assessed in an en face subcostal view (between coronal and sagittal planes at the level of the AV valve leaflet tips).

• Unbalanced AVSD: In this variation, one ventricle is significantly larger than the other. Evaluating the inlet (AV valve size) of each ventricle is especially important. Unbalanced AVSDs are frequently associated with heterotaxy, making it essential to assess abdominal situs and systemic and venous return patterns. If the stomach is located on the opposite side of the cardiac apex, heterotaxy is likely present. Additionally, the great vessel supplied by the smaller ventricle may develop stenosis; for example, if the left-sided structures are hypoplastic, there may be a risk of coarctation of the aorta.

  • In these forms, the common AV valve opening is predominantly to one ventricle rather than the other, often associated with hypoplasia of the contralateral ventricle.

    1. Unbalanced at the Atrial Level (Double Outlet Atrium): The atrial septum is malaligned with the ventricular septum, causing the right atrium to empty into both ventricles. These patients may still undergo two-ventricle repair, though they might be cyanotic.

    2. Unbalanced at the Ventricular Level: More common, occurring in about 10% of cases. 

       1. Right Dominant Form: Both atria empty into the right ventricle. 

       2. Left Dominant Form: Both atria empty into the left ventricle. 

       3. These patients are challenging to septate due to ventricular hypoplasia. The AV valve index (Left AV valve area / Total AV valve area from an en face subcostal left anterior oblique view) helps determine the degree of unbalance: <0.4 indicates right dominant, >0.6 indicates left dominant, and 0.4-0.6 tends to be balanced. The "inflow index" (color flow jet into the left ventricle) and an angle measurement can also indicate the degree of unbalance.  Other challenges include a very small mural leaflet, risking significant left AV valve regurgitation after two-ventricle repair.

Rare Variations: 

• AVCD with no primum ASD: Here, the valve is adherent to the atrial septum instead of the ventricular septum, leading to only a VSD component and no ASD. The left AV valve is still trifoliate and has a cleft. These are challenging for surgeons as there's no atrial communication to access the left AV valve. 

• Double orifice left AV valve: Can occur with AVCD. It is extremely difficult to diagnose pre-operatively, though 3D imaging helps. Surgeons may leave one orifice open or partially closed to avoid stenosis.

• Parachute left AV valve: Where all valve components attach to only one papillary muscle. This is a critical anatomical feature that influences surgical decisions and can lead to early symptoms and reoperation.

Chordal Attachments

• Evaluate for straddling, where one side of an AV valve is attached to the opposite ventricle.

• Assess for override, where blood flow from one side of an AV valve enters the other ventricle.

AV Valve Morphology

• In complete and balanced AVSD, it is crucial to define the atrioventricular valve (AVV) anatomy using an en face subcostal view (between the coronal and sagittal planes at the level of the AV valve leaflet tips).

• AV valve anatomy is classified using the Rastelli system, based on the characteristics of the anterosuperior bridging leaflet.

  • Rastelli Type A (“Attached”) – The anterior and anterolateral bridging leaflets are equal in size and attach medially to the crest of the interventricular septum (IVS).

  • Rastelli Type B (“Bridge”) – The anterior bridging leaflet is partially divided and does not attach to the IVS. Instead, it connects to a right ventricular papillary muscle arising from the RV septal surface (associated with a short moderator band).

  • Rastelli Type C (“Cut”) – The anterolateral leaflet is small and displaced toward the right side, while the anterior bridging leaflet appears to "float" above the IVS.

• Important Note: This classification should not be attempted from an apical four-chamber view.

Physiological Impact of AVSD

• In the absence of pulmonary stenosis (PS), an atrioventricular septal defect results in left-to-right shunting at both the atrial and ventricular levels.

• The ventricular septal defect (VSD) is large, leading to equalized right and left ventricular pressures and elevated pulmonary artery pressures (PAH).

• Once pulmonary vascular resistance drops, there is increased pulmonary blood flow, resulting in pulmonary overcirculation and congestive heart failure.

• The physiological impact varies depending on the complexity of the AVSD and associated anomalies.

Echocardiographic for AVSD Assessment

Common Findings: The characteristic common annulus and elongation of the left ventricular outflow tract (LVOT), leading to a "goose neck deformity". This elongation makes the LVOT vulnerable to obstruction, even post-repair. The AV valve to apex length is shorter than the apex to aortic length, unlike in a normal heart.

1. Imaging the AV Canal

• Define AV valve morphology and attachments.

• Assess AV valve balance, including commitment of the AV valve to each ventricle and ventricular size.

  • Use the subcostal "in-between" en face view (~4:30 pm notch position).

• Evaluate for AV valve insufficiency and determine the mechanism of regurgitation.

• Assess the subvalvular apparatus, with a focus on papillary muscle anatomy, particularly in cases where closing the cleft mitral valve might cause mitral stenosis.

• Obtain an en face view of the AV valve.

2. Hemodynamic Assessment

• Assess the global cardiac function and impact of volume and pressure overload on both ventricles.

• Evaluate left ventricular systolic function:

  • M-mode fractional shortening (FS) and biplane ejection fraction (EF) using Simpson’s rule.

• Measure left atrial (LA) size using the LA/AO ratio in the parasternal short-axis (PSSA) view.

  • LA/AO ratio >1.6 suggests a significant left-to-right shunt.

• Assess left ventricular (LV) size using:

  • M-mode LV end-diastolic dimension (LVEDd) Z-score.

  • LV end-diastolic volume (LVEDV) via biplane EF measurement.

• Estimate pulmonary artery (PA) pressure:

  • Measure the pressure gradient between LV and RV using Doppler assessment of VSD flow velocity (use best-aligned Doppler signal).

  • Estimate PA pressure via peak TR velocity (for RV and PA systolic pressures) and peak PR velocity (for mean PA pressure assessment).

  • Subxiphoid View:

    1.     Provides an excellent view of the atrial communication and the common AV valve en face.

    2.     Helps assess balance of flow across the valve.

    3.     Crucial for determining Rastelli classification.

    4.     The subxiphoid left anterior oblique view is particularly useful for visualizing the "goose neck deformity" and the en face left AV valve.

    5.    The subxiphoid short axis view helps determine if a patient has AVCD by showing the orientation of the left AV valve, which is perpendicular to the ventricular septum, unlike the parallel orientation of a normal mitral valve.

  • Apical Four-Chamber View:

    1. Provides a clear sense of the AV septal defect and the AV valve's position within it.

    2. Allows assessment of AV valve function and regurgitation jets.

    3. Helpful in determining balance and the size of atrial and ventricular components, as well as ventricular size and function.

  •  Apical Five-Chamber View: Offers a good look at the left ventricular outflow tract.

  • Parasternal Views:

    1. Long Axis View: Can be difficult to distinguish atrial vs. ventricular shunting levels. Useful for assessing outflow tracts and septal flattening (to estimate RV pressure).

    2. Short Axis View: Provides an excellent look at the left AV valve cleft and papillary muscle anatomy, identifying abnormalities like parachute left AV valves. It highlights the trifoliate nature of the left AV valve in AVCD, unlike the normal bifoliate mitral valve. The "cleft" itself is the space between the superior and inferior bridging leaflets.

  • Suprasternal View: Used to assess the aortic arch and ductus arteriosus.

  • 3D Echocardiography: Has significantly enhanced understanding of AVCD, providing a "moving pathologic picture". It clearly visualizes the common annulus, bridging leaflets, and helps in identifying double orifices or other complex anatomies.

  • Assessing Unbalance: In addition to the AV valve index, the inflow index (how much flow gets into the left ventricle) and specific angle measurements are used to assess the degree of unbalance. MRI can further assess left ventricular volumes, cardiac index, and inflows, especially in complex or unbalanced cases.

Detection of Associated Lesions

• Left Ventricular Outflow Tract Obstruction (LVOTO)

  • In complete AV canal defects (CAVC), the LVOT is elongated and narrowed, resulting in a gooseneck deformity on angiography.

  • The aortic valve remains in its normal position, but the bridging leaflets create a shallow LVOT, increasing obstruction risk.

  • Potential causes of LVOTO:

    • Accessory left-sided AV valve attachments to the septum (more common in Rastelli Type A).

    • Discrete subaortic membrane formation.

    • Anomalous or prominent anterolateral papillary muscle.

• Right Ventricular Outflow Tract Obstruction (RVOTO) – Evaluate for Tetralogy of Fallot (TOF).

• Patent Ductus Arteriosus (PDA) – If PA pressures are systemic, the PDA may be difficult to detect.

• Aortic Arch Anomalies – Assess for coarctation, especially in RV-dominant CAVC.

• Septal Defects – Evaluate the atrial and ventricular septum for additional defects.

• Systemic and Pulmonary Venous Connections

  • Assess for left superior vena cava (L-SVC) and partial anomalous pulmonary venous return.

  • Rule out coronary sinus septal defect.

Postoperative Assessment

• Residual Septal Defects

  • Evaluate for any residual atrial or ventricular septal defects, particularly in early postoperative studies.

• Tricuspid Valve Assessment

  • Check for tricuspid stenosis from apical views using pulsed and CW Doppler.

  • Assess tricuspid regurgitation from apical and parasternal long-axis views.

  • Estimate right ventricular pressure using TR jets (from apical and parasternal views).

• Mitral Valve Evaluation

  • Assess for mitral stenosis and regurgitation; if stenosis is present, measure the mean inflow gradient.

  • Perform an en-face parasternal short-axis assessment of the mitral valve to determine the mechanism of regurgitation (e.g., residual cleft).

  • Use color Doppler and 2D imaging side-by-side. Consider 3D imaging for a detailed mitral valve assessment.

• Subaortic Region Evaluation

  • Assess for subaortic obstruction using imaging, pulsed Doppler, and color Doppler.

• Aortic Valve Function

  • Evaluate for aortic regurgitation.

Presentation by Elissa Remmer and Stephanie Mardakis on Atrio-Ventricular Septal Defect

Case 1

Case 2

Pathological Specimens

Maude Abbott Medical Museum